This invention relates to chemically milling of aluminum and in particular to a method and composition for etching and chemically milling lithium containing aluminum alloys.
Applicants are aware of the following U.S. Pat. Nos.: 2,177,751; 2,434,021; 2,647,864; 2,650,157; 2,890,944; 2,940,838; 3,033,795; 3,052,482; 3,171,767; 3,230,172; 3,253,968; 3,301,719; 3,367,874; 3,479,293; 4,264,377; 4,284,468; and 4,592,854. The disclosures of the above patents are incorporated by reference herein.
There are numerous methods and compositions known for etching metals. These prior methods and compositions are severely limited in effectiveness when applied to the lithium containing aluminum alloys, which have recently become available in commercially useful quantities. As a result, the art is generally using acid etchants for lithium containing aluminum alloys, and for alloy 2090 in particular. Acid etch solutions have inherent deficiencies when applied to aluminum alloys and it is desirable to caustic etch aluminum lithium alloys, if an effective etchant and method can be developed.
Lithium containing aluminum alloys have only recently been available to industry in commercial quantities. These alloys are highly desirable, particularly in the aircraft industries, due to their low density and high specific strength, that is, their very high strength to weight ratio. These alloys can have a significant weight saving advantage over conventional aircraft aluminum alloys, such as aluminum alloy 7075 and similar aluminum alloys. This weight and strength advantage makes the aluminum lithium alloys highly desirable since use of these alloys can result in a significant structural weight reduction without any loss in strength. This weight reduction in the aircraft industry translates into a greater payload for the same amount of fuel expended or a greater range for an equivalent payload. These are highly significant factors influencing the profitability and utility of the use of these alloys in the aircraft industry. However, for the alloy to have full utility in the aircraft industry, and in other industries, it must be susceptible to chemical milling so that excessive machining costs are not incurred in producing strong, lightweight parts.
The most desirable etching system for etching conventional aluminum alloys has been a caustic etch system. Caustic etch systems have an inherent advantage over acid etch systems for aluminum. The solubility of aluminum in acids is very low, typically from about 15 to 17 grams per liter at normal operating temperatures. The solubility of aluminum in caustic etchant may be as high as 70 grams per liter, or perhaps slightly more, at normal operating temperatures. Since caustic etchants have the greatest inherent solubility advantage, it would be desirable to etch aluminum lithium alloys in caustic baths. However, conventional caustic baths have proved highly unsatisfactory for lithium containing aluminum alloys. In particular, conventional caustic etch baths have produced highly irregular, coarse etched surfaces which are entirely unsuitable both from a cosmetic and a strength stand point. The surface produced by conventional caustic etchant on lithium containing aluminum alloys is irregular, rough, pitted and has a high degree of stress concentrating structures. These structures reduce the effective strength of the material and eliminate any inherent strength to weight advantage which lithium containing aluminum alloys have over conventional alloys. As a result, the industry regards caustic etchants as unsuitable for lithium containing aluminum alloys and as especially unsuitable for alloy 2090, the preferred alloy for many advanced designs. The art currently uses acid etchant systems, in spite of the low solubility of aluminum in acid and the additional disadvantage of having to find suitable methods of disposing of toxic acid waste products from the acid etchant baths.
Applicants' have discovered a caustic etchant and a method of etching aluminum lithium alloys using the caustic etchant that is effective to produce a smooth chemically milled surface which is free of stress concentrators and has a highly desirable cosmetic appearance. Applicants' caustic etchant has a high solubility for aluminum which permits an etchant solution to be used in an efficient manner to mill a large quantity of lithium containing aluminum material. Applicants' etchant does not contain any highly toxic chemicals so the spent etchant bath can be disposed of without creating an environmental hazard. The composition contains a caustic etchant material such as sodium hydroxide or other inorganic caustic etchant and a high proportion of inorganic sulfide, such as sodium sulfide or equivalent reactive sulfides. The bath may contain a nominal amount of aluminum to increase the activity of the etchant bath. Preferably, a chelating agent, such as an amine chelating agent, preferably triethanolamine or an equivalent chelating agent, is added to the etchant bath. In addition to these chelating agents, just described, we have found that small amounts of ethylenediaminetetraacetic acid (EDTA) and a soluble carbonate, such as sodium carbonate, are especially beneficial to improving fillet radii and undercut factors. The addition of the chelating agent eliminates the need for adding dissolved aluminum metal to the etchant bath as an activating agent, further extending the useful life of the etchant bath. Dissolved aluminum may be added, if desired. The soluble carbonate, chelating agent and EDTA, in combination, act as etch rate controlling compounds which provide a uniform etch of the alloy, preventing formation of stress concentrating structures and insuring a smooth, desirable milled surface.
Applicants' method of chemically etching or milling lithium containing aluminum alloys includes the preparation of sufficient quantity of the etchant in a suitable container or tank. The aluminum lithium parts to be etched or milled are immersed in the tank for the appropriate period of time to remove the desired amount of material. The components of the etchant, such as sodium hydroxide and sodium sulfide, can be replenished as they are consumed during etching or milling of the aluminum lithium alloy. Typically, the caustic chemical milling bath contains an inorganic hydroxide, such as sodium hydroxide, in an amount between about 125 to 300 grams per liter, contains an inorganic sulfide, such as sodium sulfide, in an amount between about 35 to 145 grams per liter and a complexing agent, such as triethanolamine, in an amount between about 0.5 to 4 percent by volume. The bath may also contain ethylenediaminetatraacetic acid in an amount between about 1.0 to 10.0 grams per liter and sodium carbonate in an amount between about 2.0 and 20.0 grams per liter of etchant. The remainder of the composition is essentially water. The inorganic sulfide, as noted above, is present in an amount sufficient to provide a sulfur concentration (as sulfide ion) in the bath of from about 15-60 grams per liter.
A preferred composition for the bath is between about 190 to 220 grams per liter of inorganic hydroxide, between about 100 to 120 grams per liter of inorganic sulfide (about 40-50 grams per liter of sulfur, as sulfide) and about 2 percent by volume of complexing agent. A preferred concentration for the additional complexing agent EDTA is between about 1.0 to 5.0 grams per liter, and the preferred concentration of sodium carbonate is between about 2.0 to 8.0 grams per liter of etchant.
Applicant's invention has been found highly effective at chemically milling aluminum lithium alloys, including the hard to mill alloy 2090. To date this etchant has not been as effective at milling conventional aluminum alloys, such as alloy 7075, as are conventional caustic etchant systems. The sodium carbonate, or an equivalent carbonate, is especially helpful, as applicants have discovered, at improving the performance of the bath in etching fillet radii and in reducing undercut, and also substantially increasing the effective life of the bath. The added carbonate greatly reduces replating of lithium (or lithium complex) on the workpiece, which would interfere with the etch rate. The added carbonate effectively removed dissolved lithium from the etch path by precipitating low solubility lithium carbonates and complexes.
The invention may be more fully understood by referring to the following Description of the Drawings and Description of the Preferred Embodiments.